1. Field of the Invention (Technical Field)
The present invention relates to melanocortin receptor-specific cyclic peptides, particularly cyclic peptides selective and specific for the melanocortin-1 receptor, which may be used in the treatment of melanocortin-1 receptor-mediated or responsive diseases, indications, conditions and syndromes.
2. Description of Related Art
The following discussion refers to a number of publications by author(s) and year of publication, and that due to recent publication dates certain publications are not to be considered as prior art vis-a-vis the present invention. Discussion of such publications herein is given for more complete background and is not to be construed as an admission that such publications are prior art for patentability determination purposes.
A family of melanocortin receptor types and subtypes has been identified. Receptor types include melanocortin-1 (MC-1) receptor (MCR-1), commonly known to be expressed in normal human melanocytes and on melanoma cells, but which is also reported to be expressed in various other cells, including those involved in immune responses, such as monocytes, neutrophils, lymphocytes, dendritic cells, natural killer (NK) cells and endothelial cells. See generally, Kang, L., et al., “A selective small molecule agonist of melanocortin-1 receptor inhibits lipopolysaccharide-induced cytokine accumulation and leukocyte infiltration in mice,” J. Leuk. Biol. 80:897-904 (2006), and references cited therein. A variety of human MCR-1 subtypes and variants are known, including those disclosed in U.S. Pat. Nos. 6,693,184 and 7,115,393. In addition to MCR-1, other melanocortin receptor types include melanocortin-2 receptor (MCR-2) for ACTH (adrenocorticotropin), expressed in cells of the adrenal gland, melanocortin-3 and melanocortin-4 (MC-4) receptors (MCR-3 and MCR-4), expressed primarily in cells in the hypothalamus, mid-brain and brainstem, and melanocortin-5 receptor (MCR-5), expressed in a wide distribution of peripheral tissues.
The primary endogenous melanocortin agonist is the cyclic α-melanocyte-stimulating hormone (“α-MSH”) peptide. Melanocortin receptor-specific peptides generally contain the central tetrapeptide sequence of native α-MSH, His6-Phe7-Arg8-Trp9 (SEQ ID NO:1), or a mimetic or variation thereof, including various substitutions at one or more positions (see, e.g., Hruby, V. J., et al., “Alpha-Melanotropin: the minimal active sequence in the frog skin bioassay,” J. Med. Chem., 30:2126-2130 (1987); Castrucci, A. M. L., et al., “Alpha-melanotropin: the minimal active sequence in the lizard skin bioassay,” Gen. Comp. Endocrinol., 73:157-163 (1989); Haskell-Luevano, C., et al., “Discovery of prototype peptidomimetic agonists at the human melanocortin receptors MC1R and MC4R,” J. Med. Chem., 40:2133-2139 (1997); Holder, J. R., et al., “Structure-activity relationships of the melanocortin tetrapeptide Ac-His-DPhe-Arg-Trp-NH2. 1. Modifications at the H is position,” J. Med. Chem., 45:2801-2810 (2002); Abdel-Malek, Z. A., et al., “Melanoma prevention strategy based on using tetrapeptide α-MSH analogs that protect human melanocytes from UV-induced DNA damage and cytotoxicity,” FASEB J., 20:E888-E896 (2006); Bednarek, M. A., et al., “Cyclic analogs of α-melanocyte-stimulating hormone (αMSH) with high agonist potency and selectivity at human melanocortin receptor 1b,” Peptides, 29:1010-1017 (2008); Koikov, L. N., et al., “Analogs of subnanomolar hMC1R agonist LK-184 [Ph(CH2)3CO-His-D-Phe-Arg-Trp-NH2]. An additional binding site with the human melanocortin receptor 1?” Bioorg. Med. Chem. Lett. 14:3997-4000 (2004); and Abdel-Malek, Z. A., et al., “Alpha-MSH tripeptide analogs activate the melanocortin 1 receptor and reduce UV-induced DNA damage in human melanocytes,” Pigment Cell Melanoma Res. 22:635-44 (2009)).
Peptides or peptide-like compounds asserted to be specific for one or more melanocortin receptors are disclosed in U.S. Pat. Nos. 5,576,290, 5,674,839, 5,683,981, 5,714,576, 5,731,408, 6,051,555, 6,054,556, 6,284,735, 6,350,430, 6,476,187, 6,534,503, 6,600,015, 6,613,874, 6,693,165, 6,699,873, 6,887,846, 6,951,916, 7,008,925, 7,049,398, 7,084,111, 7,176,279, 7,473,760, and 7,582,610; in U.S. published patent application Publication Nos. 2001/0056179, 2002/0143141, 2003/0064921, 2003/0105024, 2003/0212002, 2004/0023859, 2005/0130901, 2005/0187164, 2005/0239711, 2006/0105951, 2006/0111281, 2006/0293223, 2007/0027091, 2007/0105759, 2007/0123453, 2007/0244054, 2008/0039387, and 2009/0069242; and in international patent applications nos. WO 98/27113, WO 99/21571, WO 00/05263, WO 99/54358, WO 00/35952, WO 00/58361, WO 01/30808, WO 01/52880, WO 01/74844, WO 01/85930, WO 01/90140, WO 02/18437, WO 02/26774, WO 03/006604, WO 2004/046166, WO 2005/000338, WO 2005/000339, WO 2005/000877, WO 2005/030797, WO 2005/060985, WO2006/048449, WO 2006/048450, WO 2006/048451, WO 2006/048452, WO 2006/097526, WO 2007/008684, WO 2007/008704, WO 2007/009894 and WO 2009/061411.
Notwithstanding the intense scientific and pharmaceutical interest in melanocortin receptor-specific peptides, there remains a need for highly selective and specific MCR-1 agonist peptides for use in pharmaceutical applications. It is against this background that the present invention was made.